A battery that is mounted to an electric automobile or the like is insulated from the vehicle body. When a ground fault occurs with the battery, the danger of electric shock due to the battery becomes high. Accordingly, circuits of various types have been proposed for detecting when the battery suffers a ground fault. For example, in Patent Document #1, there is described an insulation deterioration detection device in which, along with an AC oscillation circuit being connected to a high voltage battery via a first capacitor, a comparison circuit is also connected to the high voltage battery via a second capacitor. This insulation deterioration detection device detects that electrical leakage of the high voltage battery (that is insulated) is occurring by the comparison circuit comparing together an AC signal from the AC oscillation circuit and an AC signal that is inputted via the second capacitor.